Polymer electrode membrane fuel cell (pemfc) sensor

ABSTRACT

A polymer electrolyte membrane fuel cell (PEMFC) sensor includes an anode and a cathode with a polymer electrolyte disposed therebetween. The anode and cathode are configured with asymmetric catalyst loadings, such that the catalyst loading on the cathode is lower than that of the anode. Accordingly, due to the reduction of the amount of catalyst utilized, the cost of fabricating the sensor is substantially reduced.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/472,770 filed Mar. 17, 2017, the contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The embodiments disclosed herein relate to electrochemical sensors. Inparticular, the embodiments disclosed herein relate to electrochemicalsensors utilizing a polymer electrolyte. More particularly, theembodiments disclosed herein relate to electrochemical sensors formed aspolymer electrolyte membrane fuel cells (PEMFC) that utilize electrodeshaving variable catalyst loading.

BACKGROUND

Accurate, rapid and low-cost detection and quantification of an analytegas is required in many applications, such as in the analysis of ethanolgas or acetone gas or hydrogen chloride gas concentration in humanbreath, the analysis of foods and beverages, the analysis of gases inoil, gas and petrochemical industries, and in agricultural andenvironmental analyses. For example, an ethanol gas concentrationmeasurement of a person's exhaled breath is essential for thedetermination of a blood alcohol concentration to identify drunkdrivers. Another example is the measurement of acetone in a person'sexhaled breath for diabetic persons. While technologies exist to measureblood alcohol concentration—such as gas chromatography, infrared, andsemiconductor techniques—fuel cell-based sensors have been more popularfor performing breath ethanol measurements due to their superioraccuracy, linearity, sensitivity and selectivity, portable field-basedsize, moderate-cost, and rapid response time allowing the assessment ofvehicle drivers to be expedited. Although current fuel cell sensorsprovide superior performance to measure ethanol gas concentration overother technologies, such fuel-cell sensor technology has not beenimproved for some time. That is, while significant progress has beenmade in the areas of nano-technology, catalysts, and fuel cells ingeneral, currently available fuel cell sensors have not been similarlyadvanced. In particular, the embodiments of the sensor disclosed hereinmakes advances in the efficient use of expensive catalysts in theelectrodes of fuel cell sensors, and, separately and distinctly,provides fuel cell sensors that avoid the use of liquid phosphoric andsulfuric acid electrolytes, which pose a serious safety issue to usersof many fuel cell sensors. Given the high cost and safety concerns ofthe prior art fuel cell sensors, their adoption and use has beenlimited, and the embodiments of the sensor disclosed herein will improvetheir chances of being employed safely and cost effectively.

SUMMARY

Therefore, it is one aspect of the various embodiments disclosed hereinto provide a polymer electrolyte membrane fuel cell (PEMFC) sensor thatutilizes asymmetric catalyst loadings, such that the catalyst loading onthe cathode is lower than that of the anode, so as to reduce the overallamount of catalyst needed by the sensor, resulting in a reduced overallcost of the sensor.

It is another aspect of the various embodiments disclosed herein toprovide a polymer electrolyte membrane fuel cell (PEMFC) sensor thatincludes an anode; a cathode that is configured to react with air; and apolymer electrolyte disposed between the anode and cathode, wherein theanode and cathode have a variable loading of a catalyst thereon.

It is yet another aspect of the various embodiments disclosed herein toprovide an electrochemical sensing device that includes a PEMFC sensorthat includes an anode; a cathode that is configured to react with air;and a polymer electrolyte that is disposed between the anode andcathode, wherein the anode and cathode have a variable loading of acatalyst thereon; and wherein the electrochemical sensing deviceincludes a chamber body that has a cavity disposed therein; a first portthat is in communication with the cavity, with the first port configuredto receive the air therethrough; and a second port in communication withthe cavity, the second port being configured to receive an analytetherethrough; wherein the PEMFC is positioned within the cavity, suchthat the air and the analyte are separated within the cavity.

It is still another aspect of the various embodiments disclosed hereinto provide a method of forming a polymer electrolyte membrane fuel cell(PEMFC) sensor, that includes the steps of providing an electrolyte, ananode, and a cathode, wherein the anode and the cathode have a variableloading of catalyst thereon; and hot pressing the electrolyte, the anodeand the cathode together to form the PEMFC sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the various embodimentsdisclosed herein will become better understood with regard to thefollowing description, appended claims, and accompanying drawingswherein:

FIG. 1 is an exploded view of a polymer electrolyte membrane fuel cell(PEMFC) sensor in accordance with one or more embodiments disclosedherein;

FIG. 2 is a schematic cross-sectional view of an example PEMFC sensorfunctioning to sense ethanol as an analyte gas, for example, in humanbreath;

FIG. 3 shows the current density measurement of ethanol=0.05% usingdifferent Pt loading on the electrodes of exemplary PEMFC sensors, withgraph (a) showing results for various PEMFCs employing electrodes withidentical catalyst loading; graph (b) showing results for various PEMFCswith electrodes with variable catalyst loading, i.e., with lowercatalyst loading on the cathode as compared to the anode; and graph (c)helping to show concepts of peak time, decay/recover time, peak currentand charge density (area underneath of current curve);

FIG. 4 shows the performance of PEMFC sensors with identical Pt loadingon electrodes as a function of % BAC (i.e. ethanol) and their linearityresponse, with graph (a) showing peak current density of sensors withdifferent Pt loading; graph (b) showing charge density of sensors withdifferent Pt loading; graph (c) showing regression coefficient of peakcurrent densities for sensors operating from BAC 0.005-0.1% and0.005-0.2%; and graph (d) showing regression coefficient of chargedensities for sensors operating from BAC 0.005-0.1% and 0.005-0.2%;

FIG. 5 shows the performance of PEMFC sensors with variable catalystloading in accordance with the embodiments disclosed, showing low Ptloaded cathode (20% Pt/C-0.03 mg/cm²) as a function of % BAC and theirlinearity response, with graph (a) showing peak current density ofsensors with different anode Pt loadings; graph (b) showing chargedensity of sensors with different anode Pt loading; graph (c) showingregression coefficient of peak current densities for sensors operatingfrom BAC 0.005-0.1% and 0.005-0.2%; and graph (d) showing regressioncoefficient of charge densities for sensors operating from BAC0.005-0.1% and 0.005-0.2%;

FIG. 6 shows the performance of a commercial alcohol sensor as afunction of % BAC and its linearity response, with graph (a) showingpeak current density; graph (b) showing charge density; graph (c)showing an SEM micrograph (magnification: ×500); and graph (d) showingan energy-dispersive X-ray spectroscopy (EDS) analysis; and

FIG. 7 shows the repeatability analysis of the best developed sensor(i.e. 40% Pt/C-0.30 mg cm-2 anode and 20% Pt/C-0.03 mg cm-2 cathode),wherein the solid line is the fresh sensor testing, and the dash linesrepresent test runs after the sensor sat idle for 2 months in the lab.

DETAILED DESCRIPTION

A polymer electrolyte membrane fuel cell (PEMFC) sensor, hereinaftersensor 10, is shown in FIGS. 1 and 2. In particular, the sensor 10 isformed as a laminated or layered structure having a polymer electrolytelayer 20 that is disposed between an anode layer 30 and a cathode layer40 to form the sensor 10. To facilitate the flow of electrical currentfrom the sensor 10 during its operation, an anode current collectorlayer 50 is positioned in electrical communication with the anode layer30 and a cathode current collector layer 60 is positioned in electricalcommunication with the cathode layer 40. It should be appreciated thatthe anode layer 30, the electrolyte layer 20, and the cathode layer 40are directly adjacent layers that are in physical contact with eachother as generally shown in the schematic of FIG. 2. During operation ofthe sensor 10 oxygen of ambient air interacts with the cathode layer 40,while an analyte gas or liquid that is being investigated/analyzed (ordetected/quantified) interacts with the cathode layer 40. The analytegas, under the influence of the catalyst, produced electrons at theanode layer 30, while the oxygen in the ambient air at the cathode formswater molecules with those generated electrons. In the specific exampleof FIG. 2, the analyte gas is ethanol, which produced 2 electrons at theanode—appropriate chemical reactions are shown in the schematic of FIG.2. The electrons generate an electrical current produced by the sensor10 that flows from the anode 30 to the cathode 40. That is, electricalcurrent generated by the anode layer 30 flows from the current collectorlayer 50 to a suitable electrical current detector (to be discussed),which measures one or more parameters, such as magnitude of theelectrical current produced, in order to identify the concentration ofthe analyte being investigated, whereupon the electrical current returnsto the cathode current collector 60 and back to the cathode 40.

The analyte can be virtually any gas or liquid commonly analyzed in theart. non-limiting examples include acetic acid, acetone, ammonia,benzene, butadiene, carbon dioxide, chlorine, ethanol, ethylene oxide,formaldehyde, hydrochloric acid, hydrogen chloride, hydrogen peroxide,hydrogen sulphide, iso-propanol, mercaptan, methanol, methylenechloride, MTBE, Nitrogen Dioxide, nitrous gases, oxygen, ozone,perchloroethylene, petroleum hydrocarbons, phosgene, phosphine, propane,styrene, sulfur dioxide, toluene, training chip, trichloroethylene,vinyl chloride, and xylene.

The polymer electrolyte layer of the sensor 10 can be virtually anysolid polymer electrolyte. In particular embodiments, the solid polymerelectrolyte is selected from fluoropolymer (PFSA) Nafion™ (The ChemoursCompany, a DuPont product) While Nafion™ is an ionomer with aperfluorinated backbone like Teflon, there are many other structuralmotifs used to make ionomers for proton-exchange membranes. Many usepolyaromatic polymers, while others use partially fluorinated polymers.

In particular embodiments, the solid polymer electrolyte is a sulfonatedtetrafluoroethylene based fluoropolymer-copolymer, such as Nafion™ or aTitania-Nafion™ composite material. In some embodiments, Nafion™ isprovided as one or more sheets that are activated by the followingsteps: 1) immersion of the one or more Nafion™ sheets in boiling 3% H₂O₂aqueous solution for about 1 hour; 2) rinsing of the one or more Nafion™sheets with distilled de-ionized (DDI) water 1 or more times, such as 3,and boil the one or more sheets in DDI water for about 1 hour; 3)immersing the one or more Nafion™ sheets in boiling 1 mol/L H₂SO₄aqueous solution for 1 hour; and 4) rinsing the one or more sheets withDDI water 1 or more times, such as 3, and keep the one or more sheets ofNafion™ in the DDI water until fabrication of the sensor.

There are various types of Nafion™ that can be employed, includingNafion™ 115, Nafion™ 117, Nafion™ 211, Nafion™ 212, Nafion™ XL, Nafion™1110, Nafion™ 438.

The anode layer 30 and the cathode layer 40 are electrically conductive,but are electrically isolated from each other by the electrolyte layer20. In some embodiments, the anode and cathode layers 30, 40 are gasdiffusion electrodes (GDE). The electrodes may be chosen from virtuallyany suitable gas diffusion electrode given the analyte of interest, thegas diffusion electrode having any suitable catalyst loaded thereon. Insome embodiments, the gas diffusion electrodes may be formed of Vulcancarbon-supported catalyst.

The catalyst may be chosen from virtually any suitable catalyst giventhe analyte of interest and due consideration to the reactions producingthe electrons at the sensor 10. In some embodiments, the catalyst isselected from the group consisting of platinum, palladium, ruthenium,rhodium, nickel, gold, titanium, silver, copper, tin, zinc, lead,iridium, vanadium, cobalt, manganese, Iron, aluminum, and other metalbased catalysts (metal oxides, metal chloride, etc), and combinationsthereof.

The catalyst will be loaded at the anode at any suitable catalystloading as generally known and practiced for a given analyte. Theembodiments of the sensor disclosed herein advances the art by theimplementation of variable catalyst loading at the cathode and anode andoptimized micro-structure or nano-structure of electrodes and theircatalyst layer(s) and different patterns for catalysts distribution onthe catalyst layer. As will be described below, the sensor 10 isemployed in a electrochemical analysis device 100 (FIG. 1), with ambientatmosphere at the cathode. With the oxygen of the ambient atmospheregenerally being at significantly higher concentration than the analyteat the anode (e.g. 21 volume % O₂ in atmosphere), it has been found thatthe catalyst loading at the cathode can be significantly lower than atthe anode and yet a sensor with good sensitivity an linearity ofresponse can be achieved. This is termed herein “variable catalystloading”, wherein the amount of catalyst loaded at the anode is not thesame as the amount loaded at the cathode. It is believed that this is anovel practice in PEMFC sensors.

In some embodiments, the catalyst is loaded by employingcatalyst-on-carbon. For example, platinum on carbon (Pt/C) can be used,or rhodium on carbon, etc. Generally known and commercially availablecatalyst-on-carbon can be employed, or catalyst-on-carbon with a desiredloading can be manufactured for use. Specific amounts ofcatalyst-on-carbon are shown herein through the proof of conceptprovided in the experimental section.

With the understanding that a variable loading of catalyst is possiblein PEMFC sensors with the ability to produce acceptable sensorfunctioning, a series of sensors and experiments thereon can bedeveloped to choose acceptable functioning sensors for a givenapplication (i.e., given analyte).

In some embodiments, the catalyst is loaded to the anode using at from5% or more to 100% or less catalyst-on-carbon (wherein 100% connotespure catalyst without activated carbon support; for example, pureplatinum back), loaded at from 0.1 or more to 5.0 or less mg/cm². Insome embodiments, the catalyst is loaded at the cathode using at from 5%or more to 100% or less catalyst-on-carbon, loaded at from 0.01 to 4.0mg/cm².

In some embodiments, the catalyst is loaded to the anode using at from20% or more to 60% or less catalyst-on-carbon, loaded at from 0.2 to 0.4mg/cm². In other embodiments, the catalyst is loaded to the anode usingat from 30% or more to 50% or less catalyst-on-carbon, loaded at from0.25 to 0.35 mg/cm². In some embodiments, the catalyst is loaded to theanode using at from 35% or more to 45% or less catalyst-on-carbon,loaded at from 0.25 to 0.35 mg/cm².

In some embodiments, the catalyst is loaded at the cathode using at from5% or more to 30% or less catalyst-on-carbon, loaded at from 0.01 to 2mg/cm². In some embodiments, the catalyst is loaded at the cathode usingat from 10% or more to 30% or less catalyst-on-carbon, loaded at from0.01 to 1 mg/cm². In some embodiments, the catalyst is loaded at thecathode using at from 15% or more to 25% or less catalyst-on-carbon,loaded at from 0.02 to 0.05 mg/cm².

In a some embodiments, the sensor senses ethanol as the analyte, andcatalyst is loaded to the anode using at from 35% or more to 45% or lesscatalyst-on-carbon, loaded at from 0.25 to 0.35 mg/cm²; and the catalystis loaded at the cathode using at from 15% or more to 25% or lesscatalyst-on-carbon, loaded at from 0.02 to 0.05 mg/cm². In a specificembodiment, the sensor senses ethanol, and catalyst is loaded to theanode using 40% catalyst-on-carbon, loaded at from 0.3 mg/cm²; and thecatalyst is loaded at the cathode using 20% catalyst-on-carbon, loadedat 0.03 mg/cm².

In some embodiments, the amount of catalyst on the anode is ten timeshigher than the amount of catalyst on the cathode. In other embodiments,the amount of catalyst on the anode is twenty times higher than theamount of catalyst on the cathode.

The anode current collector 50 and the cathode current collector 60 areformed of any suitable electrically conductive material, such asstainless-steel, copper, aluminum, or gold for example. The anode andcathode current collectors 50, 60 include one or more ports or passages70 that permit the flow of gas or liquid therethrough for interactionwith respective anode and cathode layers 30, 40 that are directlyadjacent thereto. In some embodiments, the anode and cathode currentcollectors 50, 60 may be formed as a mesh, or as any other gas/liquidporous material that is electrically conductive. For example, the meshmay comprise stainless-steel, such as 316 stainless steel.

The sensor 10 may be assembled using any suitable process. For example,in some embodiments, the electrolyte layer 20 and electrode layers 30,40may be joined together to form the sensor 10 using a hot-presstechnique. However, in other embodiments, the electrolyte layer 20 andthe electrode layers 30-40 of the sensor 10 may be maintained inoperative communication with each other using mechanical compression,such as that provided by a clamp for example.

In some embodiments, the sensor 10 may be utilized in an electrochemicalanalysis device 100 for use in measuring, detecting and analyzing one ormore properties an analyte or target gas or liquid, as shown in FIG. 1.For example, the electrochemical analysis device 100 may comprise abreathalyzer comprised to measure an ethanol concentration may beutilized for detection of any target gas/liquid concentration. Thedevice 10 includes a chamber body 110, which includes a sealed cavity120 that is in fluid communication with an inlet port 130 and an outletport 140. The cavity 120 is dimensioned to receive the sensor 10therein. The chamber body 110 may be formed of any suitable material,such as polymeric material or metal for example. The chamber body 110may also include a cap 150 that is removably attached to the chamberbody 110 to access the cavity 120 and to facilitate the ease at whichthe sensor 10 is inserted and removed from the cavity 120. The cavity120 is also configured so that when the sensor 10 is inserted therein,the inlet port 130 is only in fluid communication with the anode layer30, and the outlet 140 is only in fluid communication with the cathodelayer 40. In other words, the cavity 120 is structurally configured toform 2 sub-chambers when the sensor 10 is inserted therein. As such, theinlet 130 and the cathode 40 fluidly communicate with one sub-chamber,which the outlet 140 and the anode 30 fluidly communicate with anothersub-chamber. Thus, the sub-chambers function to separate gases/liquidsflowing through the inlet 130 and the outlet 140 thereby preventing themfrom interacting with each other when interacting with the sensor 10.The chamber body 110 may also include any suitable ports to enableelectrical leads or wires 200, 210 to pass therethrough for electricalcommunication, such as by way of direct electrical attachment, with eachof the anode and cathode current collectors 50, 60 coupled to the sensor10.

In order to identify a concentration of an analyte (i.e. a gas or liquidbeing tested or studied) using the sensor 10, the current collectors 50,60 are placed in electrical communication with an electrical currentdetection device 250, which may comprise any suitable computing device,such as a portable or fixed computing device to monitor one or moreparameters of the electrical current generated by the operation of thesensor 10, such as the magnitude of an electrical current that isgenerated from the sensor 10. However, in other embodiments, thedetection device 250 may comprise a current sensor that includes thenecessary circuitry to enable wired or wireless communication with asuitable computing device, such as a portable or standalone computingdevice.

In some embodiments, the electrochemical analysis device 100 may beformed as a self-contained device that includes both the chamber housing110 and the electrical current detection device 250. Such aself-contained device may be configured as portable or fixed system.

The following discussion presents the operation of the sensor 10 when itis utilized as part of the electrochemical analysis device 100.Initially, a bias or oxidizing agent, such as a gas or liquid, includingambient air for example, is permitted to flow through the porous cathodecurrent collector 60 to interact with the cathode layer 40.Additionally, the analyte or gas/liquid being analyzed is then permittedto flow through the porous anode current collector 50 to interact withthe anode layer 30. The interaction of the analyte with the anode layer30 results in a reaction with the catalyst, provided thereon, such asplatinum (Pt) to produce free hydrogen atoms (H⁺) and free electrons(e⁻). Simultaneously, the bias or oxidizing agent is permitted to flowthrough the porous cathode current collector 60 to interact with thecathode layer 40. The interaction of the bias or oxidizing agent, suchas ambient air, with the cathode layer 40 allows the free hydrogen atoms(H⁺) that pass from the anode layer 30 and through the electrolyte layer20 to combine with the O₂ of the ambient air of the oxidizing agent toform byproduct water (H₂O). In addition, the free electrons (e⁻)generated at the anode layer 30, form an electrical current that flowsto the anode current collector 50 into the external detection circuit250, which is also electrically coupled to the cathode current collector60. That is, upon the detection of an analyte gas or liquid that iscompatible with the operation of the sensor 10, the anode layer 30generates an electrical current that flows to the electrical currentdetection device 250 that is coupled between the anode and cathodecurrent collectors 50, 60.

EXPERIMENTAL RESULTS A.) PEMFC Sensor Fabrication

In order to assess the operational performance of the PEMFC sensor 10,the following parameters were studied through experimental analysis. Themembrane electrode assembly (MEA) of PEMFC ethanol gas sensor 10comprises a solid polymer electrolyte sandwiched by two electrodes. Theelectrodes are gas diffusion electrodes (GDE) comprised of Vulcancarbon-supported Pt catalyst (HiSpec 3000 and 4000, Alfa AesarllohnsonMatthey) and Nation ionomer spread on woven carbon cloth (GDL-CT,CeTech) as the gas diffusion layer (GDL). GDEs with 10, 20, 30, and 40%platinum (pt)/carbon (c) with a Pt loading of 0.1, 0.2, 0.25, and 0.3mg/cm², respectively, were used as the electrodes of the sensor 10. The10% Pt/C, 0.1 mg/cm² Pt and 30% Pt/C, 0.25 mg/cm² Pt GDEs werecustomized in this experiment with the exact same catalyst and GDL typewith the other GDEs). The ratio of Nafion™ ionomer to Pt was heldconstant at 3:2 for all GDEs. The characteristics of a GDEs used in thisstudy are listed in Table 1.

TABLE 1 Characteristics of different gas diffusion electrodes used inthis study. Pt in Pt catalyst 1.8 cm² loading Thickness (μm) electrodeCatalyst (mg/cm²) CL⁽¹⁾ MPL⁽²⁾ GDL⁽³⁾ GDE⁽⁴⁾ (mg) 10% Pt/C 0.10 10 70310 390 0.18 20% Pt/C 0.20 10 70 310 390 0.36 30% Pt/C 0.25 9 70 310 3890.45 40% Pt/C 0.30 8 70 310 388 0.54 20% Pt/C 0.03 2 70 310 382 0.05Commercial 32 10 Electrolyte thickness; 1 mm 57.27⁽⁵⁾ sensor ⁽¹⁾CL:catalyst layer ⁽²⁾MPL: micro porous layer ⁽³⁾GDL: gas diffusion layer⁽⁴⁾GDE: gas diffusion electrode (GDE = CL + MPL + GDL) ⁽⁵⁾Commercialsensor electrode area was 1.21 cm². The amount of Pt loading forelectrode area of 1.8 cm² was extrapolated.

Nafion™ 115 with the thickness of 127 μm (Fuel Cells Etc.) was used asthe solid electrolyte membrane for fabrication of MEAs. Prior tofabrication, Nafion™ membrane requires impurity removal (cleaning) bythe following steps: Nafion™ was immersed in boiling 3 wt % H₂O₂ aqueoussolution for about 1 hour. Then, it was rinsed in de-ionized (DI) waterseveral times, followed by boiling in DI water for 1 hour. The Nafion™membrane cleaning was continued by immersing in boiling 1 M H₂SO₄aqueous solution for about another hour. Finally, the Nafion™ membranewas rinsed several times with DI water and stored in DI water at roomtemperature prior to its usage in sensor fabrication. The electrodes andactivated Nafion™ membranes were cut precisely by laser cutter machine(VLS2.30 Versa Laser) in a circular shape with the diameter of 15 mm(area is approximately 1.8 cm²). It should be noted that Nafion™membranes should be cut slightly larger than electrodes to preventshort-circuiting. Nafion™ 115 was then placed between two electrodes andwas compressed using a hot press (MTI Corporation), which applied about10 MPa (or 2.5 kN) of pressure at approximately 100° C. for about 1.5minutes to complete the MEA fabrication. The fabricated MEA was thensandwiched between two current collectors (0.01 inch thick stainlesssteel metal grid, McMaster) to improve the current collection from thesensor's electrodes. In order to ensure good electrical contact betweencurrent collectors and electrodes and to minimize gas leakage from anodechannel to cathode, the sensor chamber 110 was designed and printed by ahigh resolution 3D printing machine (Objet EDEN260V). The screw/bolt-fitchamber cap 150 was used to seal the sensor chamber cavity 120 andprovide enough pressure on the sensor current collectors 50,60 tomaintain a suitable electrical connection with the sensor 10 electrodes30,40. FIG. 2 shows the manner in which the sensor 10 is positioned inthe sensor chamber 120.

B.) Testing

Utilizing the PEMFC sensor 10 described in section A of the experimentalresults section above, various ethanol concentrations were appliedthereto to evaluate the operational performance of the PEMFC sensor 10.

Sensors 10 with different Pt loadings were subjected to 10 differentethanol concentrations from 0.005 to 0.2% to investigate theirelectrochemical performance and linearity response versus the change of% ethanol. The sensors 10 were kept at room temperature during testing.After each test with certain ethanol concentrations, the sensor 10 waswashed with pure DI water through the testing cycle for several times toremove remaining ethanol gas in tubes and housing. Removing theremaining ethanol from tubing and sensor electrodes 30,40 is essentialsince any remaining ethanol can interfere with the next run's currentmeasurement.

C.) Results of Testing

FIG. 3 shows the electrical current response of the sensors 10 withvarious electrode Pt loadings at the ethanol concentration of 0.05%.FIG. 3(a) demonstrates responses of identical electrode sensors. Theterm “identical electrode sensor” means the Pt loading for the anode andcathode of sensors 10 are exactly the same. FIG. 3(b) shares the sameconcept of FIG. 4(a) but has non-identical electrode sensors, where thecathode Pt loading is kept as low as 0.03 mg/cm² (20% Pt/C with 0.03mg/cm² Pt). It should be noted that the same current test wasaccomplished for ten % ethanol concentration values but only the resultsof an ethanol concentration=0.05% are demonstrated in this Fig. As shownin FIG. 4(c), for all case studies when the ethanol-containing gas wasinjected into the sensor's anode compartment, the current across theexternal circuit was increased sharply in a very short time (peak time).It then fell rapidly down after reaching a peak and exhibited a sluggishdecline towards the end of the cycle (decay/recovery time). The recoverycurve exhibited exponential behavior as a exp(−bt). This equation can beapplied to all fabricated sensors 10, where only a and b are changing bycatalyst loading and % ethanol concentration. The peak electricalcurrent and the area underneath the current response curve are alsodependent on the Pt loading for the sensor electrodes 30,40 and %ethanol concentration.

Three different steps may be considered for current generation in a fuelcell sensor including (i) diffusion of C2H_(s)OH and O₂ into theelectrodes catalysts, (ii) electrochemical reaction of C2H_(s)OH and O₂on the anode and cathode active sites, and (iii) H+ diffusion in solidelectrolyte polymer (Nafion™ 115). Ethanol gas is oxidized over the Ptcatalyst at the anode (reaction 1), but Pt loading on the cathode sidealso plays a role in electrical current generation by the sensor 10,since oxygen reduction (reaction 2) takes place on the cathode side.Using the same anode Pt loadings, the sensors with low Pt loading ontheir cathode exhibited lower current densities than the sensors withhigh Pt cathode loading. Nevertheless, our investigations for currentpeaks obtained from identical sensors and non-identical sensors withvery low Pt loading confirm that the value of peak current is reduced byless than 2 times on average if the catalyst loading is reducedsignificantly at the cathode 40.

D.) Identical Electrode Sensors

FIG. 4 demonstrates the sensor 10 performance with identical Pt loadingon their electrodes as a function of % ethanol concentration andexamines the linearity response of different sensors 10 by altering thecatalyst loading of the electrodes 30, 40. The peak current density ofdifferent catalyst loaded sensors is shown in FIG. 4(a). The increase ofPt/C loading from 10% to 40% raised the peak current densities, whichwas expected due to an increase in the catalyst loading on theelectrodes. Increasing the catalyst loading increases the active sitesresulting in activation polarizations reduction leading to an increasein generated current densities. In addition to the output currentdensity magnitude and sensors response linearity, the sensitivity of thesensors 10 is another important factor for the characterization of theMEA. The sensor selectivity is obtained by the slope of calibrationcurves shown in FIG. 4(a). The current density sensitivity factors(slope of calibration curves) of identical electrode sensors werecalculated by 8.51, 28.22, 41.36, and 67.64 uA/cm² for 10%, 20%, 30% and40% Pt/C loaded electrodes, respectively. The trend of the sensitivityfactors can be explained by the electrochemically active surface area(ECSA); the higher ECSA, the higher sensitivity. Although, it isreported that catalysts made from HiSpec 3000 (20% Pt/C) has higher massspecific activity than catalyst made from HiSpec 4000 (40% Pt/C) due toits smaller particle size but higher Pt loading in 40% Pt/C (0.2 mg/cm²)than 20% Pt/C (0.3 mg/cm²) led to a higher ECSA and sensitivity for 40%Pt/C (0.2 mg/cm²) catalyst.

FIG. 4(b) demonstrates the changes in the area under the currentresponse curves, which represents the charge density. The charge densitysensitivity factors of identical sensors with 10%, 20%, 30% and 40% Pt/Cloaded electrodes are calculated by 1143.6, 3611.4, 4740.6, and 7057.8uC/cm², respectively. It is believed that the area under the curve mayrepresent the ethanol concentration in the feed more accurately than byusing the peak current value. In this study, the sensitivity, and peakcurrent and charge densities follow the same trend, which shows thehighest sensitivity, current and charge densities for 40% Pt/C and thelowest ones for 10% Pt/C loading. The regression least squarecoefficient (R2) shown in FIGS. 4(c) and (d) show that all of thesensors 10 with different Pt loadings had acceptable linear responsetoward changing ethanol concentration from ethanol concentration of0.005 to 0.2%. The sensor with 20% Pt/C and 0.2 mg/cm² loading onlyshowed poor linearity at ethanol concentration between 0.005 to 0.1% anddid not perform as good as other sensors across the full range ofethanol concentration. The best linearity was obtained from the sensorwith 30% Pt/C and 0.25 mg/cm² loading, revealing more than 99%linearity. Since obtaining the peak current density is faster than thecharge density, employing the peak current density method for thesesensors to measure the ethanol concentration is preferred. Thus, thesensor is able to recover for the next use at a faster rate. It wasdiscovered that the measurement accuracy would not be changed if thepeak current method is used instead of charge density to measure theethanol gas concentration for a fresh sensor operating at roomtemperature.

E.) Non-Identical Electrode Sensors

Ethanol oxidation and hydrogen ion generation take place on the anodeside of the sensor. Since the ethanol concentration of the feed oroxidant gas is in the range of parts-per-million (ppm), the catalystloading on the anode 30 should be high enough to guarantee theoccurrence of ethanol oxidation and sensor 10 functioning.Alternatively, the oxygen concentration in air is 21 vol. %, which issignificantly higher than the ethanol concentration in the anode 30.Therefore, the Pt loading on the cathode 40 can be potentially lowerthan the anode 30. Thus, the fabrication of the sensors 10 with lower Ptloading on cathode 40 can be economically favorable, especially in massproduction of the sensor 10, without having any crucial or significanteffect on the sensor's performance. FIG. 5 demonstrates the sensors'performances with non-identical electrode 30,40 Pt loadings (low Ptloading: 20% Pt/C-0.03 mg/cm² on the cathode 40) as the function of %ethanol concentration and examines the current response and linearity ofdifferent sensors by altering the anode catalyst loading.

The peak current densities shown in FIG. 5(a) reveal that the trend ofcurrent generation of sensors with non-identical anode loading issimilar to identical electrode sensors. The current density sensitivityfactor of non-identical electrode sensors were calculated by 3.36, 2.17,18.04, and 27.51) mA/cm² for 10%, 20%, 30% and 40% Pt/C anode loadings,respectively. The sensor with 40% Pt/C and 0.3 mg/cm²/loading on theanode side showed the highest sensitivity and peak current density. FIG.5(b) shows that the charge density also follows the linear behavior byincreasing the ethanol concentration (% BAC). The charge densitysensitivity factor of this kind of sensors were calculated by 711.0,181.8, 1959.6, and 3087.0 mA/cm² for 10%, 20%, 30% and 40% Pt/C anodeloadings, respectively. The calculated least square coefficient (R²)demonstrated in FIGS. 5(c) and 5(d) shows that the sensors with low Ptloading on the cathode have a good linearity response. Only the sensorwith 20% Pt/C and 0.2 mg/cm² anode loading deviated from linearbehavior. The R² values for both peak current and charge densities arehigh.

F.) Commercial Sensor

FIG. 6 illustrates a commercial ethanol gas sensor's performance as afunction of % ethanol concentration. The commercial sensor employs verythick porous polyvinyl chloride (PVC) membrane that is loaded withliquid acid electrolyte with two thin (10 μm) identical Pt electrodes(32 mg/cm² Pt). The commercial sensor was tested in the same cellhardware with the fabricated sensors in this study. FIGS. 6(a) and 6 (b)demonstrate the peak current density and charge density generated bythis sensor, respectively. The sensitivity factor of commercial sensorfor current density and charge density were calculated by 213.85 mA/cm²and 27814.2 μC/cm², respectively. Comparison of the magnitude ofsensitivity factors, peak current and charge densities of the commercialsensor and the fabricated sensors in the lab revealed that thecommercial sensor generates up to one order of magnitude highersensitivity, current and charge densities than the sensors developed inlab. High sensitivity, current and charge densities of the commercialsensor are expected due to extremely high loading of Pt catalyst on itselectrodes. Although the generated current density of the commercialsensor is greater than the fabricated sensors in lab, the majority

TABLE 2 Comparison of best fabricated sensors in lab with a commercialsensor. Regression coefficient (R²) for peak Pt loading current density(mg/cm²) Peak current density at Current density at BAC ranges of Ptused per Sensor category Anode Cathode BAC 0.05% (μA/cm²) sensitivity0.005-0.1% 0.005-0.2% sensor (mg) Identical 0.25 0.25 2.2 41.36 0.980.99 0.90 electrode Non-identical 0.30 0.03 1.7 27.51 0.99 0.99 0.60electrode Commercial 32 32 13.7 213.85 0.93 0.99 77of the fabricated sensors exhibited slightly better linearity withchanging % BAC, especially in an ethanol concentration range of0.005-0.1%, see Table 2.

FIG. 6(c) shows the SEM micrograph of the commercial sensor with thesame magnification as the fabricated sensors in lab. The EDS analysis ofthe commercial sensor electrode illustrated in FIG. 6(d) confirms thatalmost 100 wt % of the electrode is Pt catalyst. In order to compare thedeveloped sensors with the commercial one, the best performing sensorfrom each category (i.e. identical electrode and non-identicalelectrode) was selected and listed in Table 2 along with the commercialsensor characteristics.

The criteria for selecting the best sensor was a combination of theamount of Pt loading, linearity, sensitivity and magnitude of thecurrent density generation of the sensor. Table 2 indicates that thebest sensor is the one with 40% Pt/C-0.30 mg/cm² anode and 20% Pt/C-0.03mg/cm² cathode, since it has the highest linearity response, the lowestPt loading and acceptable sensitivity. The amount of Pt used in thecommercial sensor is more than 130 times higher than that in thissensor. In addition, this sensor has 67% less Pt catalyst compared to aPEMFC sensor fabricated and tested by Kim, et. al. The advantages of thecommercial sensor over fabricated sensors are its higher sensitivity andhigher current density generation plus probably its stability. Thelatter is not the scope of this study and it is an ongoing study in ourlab. Thus, a more accurate current reading electronic circuit may berequired for the low catalyst loading sensor fabricated in lab. FIG. 7illustrates the study for durability and repeatability of the bestdeveloped sensor. The durability runs were completed after 2 months ofthe sensor 10 sitting idle in a lab environment. Getting almost similarresults from the sensor after 2 months shows that the sensor was durableand its Nafion™ membrane was still active and could preserve itsmoisture. Rerunning the sensor for 2 extra times indicated that resultsare repeatable, and the sensor could keep its accuracy for all repeatedruns.

A recent study using power-generating fuel cell electrode material asbreath alcohol sensor with 20% Pt/C and Pt loading of 0.4 mg/cm² showed97% less Pt usage compared to the commercial sensor. However, in ourstudy we developed the sensor with non-identical Pt loading electrodeswhich lead to the use of 100-130 times less Pt loading than commercialsensor due to applying cathodes with extremely low amount of Pt loading.Subsequently, we depict that with this lower amount of Pt loading,sensitivity and linearity of the results are still reliable as a breathalcohol sensor.

G). Conclusion

Solid polymer electrolyte fuel cell sensors with different Pt catalystloading on (i.e. electrodes were fabricated to measure the ethanol gasconcentration in exhaled human breath. The sensitivity, peak currentdensity and charge density for two types of sensors 10 including (i)identical electrode sensors, and (ii) non-identical electrode sensorswere measured. The results revealed that the GDE with 40% Pt/C-0.3mg/cm² loading has the highest sensitivity factor, peak current andcharge densities of all types of the sensors studied. The least squarecoefficients (R²) for almost all of the two types of sensors were foundto be acceptable for detecting ethanol concentration with very goodaccuracy in the ethanol concentration range of 0.005 to 0.2%. The R²values for peak current densities were as high as the charge densities.Thus, using the peak current density measurement method is desirable insome cases over the charge density measurement method since the lattertakes more time for completion of the sensor reading. Utilizing a low Ptcontaining cathode 40 did not have a significant adverse effect on theperformance of sensor 10, indicating that this type of sensor is morecost-effective than other types without losing accuracy. Comparison ofthe performance of the commercial sensor with the fabricated lowcatalyst loading sensors 10 indicates that sensors 10 are significantlyless costly and operate more linearly in a wider range of % ethanolconcentration. Considering the merits of a fuel cell sensor 10 (i.e.sensitivity, high current generation, high linearity response in widerange of % ethanol concentrations, and low production cost), the sensor10 with anode 30 loading of 40% Pt/C-0.3 mg/cm² and cathode 40 loadingof 20% Pt/C-0.03 mg/cm² is the most desirable. This sensor 10 requires130 times lower Pt loading compared to the commercial sensor, and 67%less Pt loading as compared to the PEMFC sensor studied by otherresearchers for ethanol gas measurement.

Using catalyst as low as 40% Pt/C-0.3 mg/cm² for anode and 20% Pt/C-0.03mg/cm² for cathode has high sensitivity to breath alcohol content. Usingcatalyst as low as 40% Pt/C-0.3 mg/cm² for anode and 20% Pt/C-0.03mg/cm² for cathode has relatively high current generation for wide rangeof % BAC from 0.005% to 0.2%. Using catalyst as low as 40% Pt/C-0.3mg/cm² for anode and 20% Pt/C-0.03 mg/cm² for cathode has highly linearresponse for wide range of % BAC from 0.05% to 0.2%. High accuracy andlinearity of current density response to alcohol content is an enablerfor using peak current density instead of charge density for alcoholcontent measurement, and asymmetric loading results in a sensor withfast response times.

What is claimed is:
 1. A polymer electrolyte membrane fuel cell (PEMFC)sensor comprising: an anode; a cathode configured to react with air; anda polymer electrolyte disposed between said anode and cathode, whereinsaid anode and cathode have a variable loading of a catalyst thereon. 2.The PEMFC sensor of claim 1, further comprising: a first currentcollector in electrical communication with said cathode; and a secondcurrent collector in electrical communication with said anode, whereinsaid first and second current collectors are porous.
 3. The PEMFC sensorof claim 2, further comprising a current sensor in operativecommunication with said first and second current collectors.
 4. ThePEMFC sensor of claim 1, wherein said variable loading of said catalystcomprises an amount of said catalyst on said cathode that is less thanan amount of said catalyst on said anode.
 5. The PEMFC sensor of claim1, wherein said catalyst is platinum (Pt).
 6. The PEMFC sensor of claim5, wherein said catalyst is disposed on a porous carbon (C) substrate.7. An electrochemical sensing device including said PEMFC sensor ofclaim 1, wherein said electrochemical sensing device further includes: achamber body having a cavity disposed therein; a first port incommunication with said cavity, said first port configured to receivethe air therethrough; and a second port in communication with saidcavity, said second port configured to receive an analyte therethrough;wherein said PEMFC is positioned within said cavity, such that the airand the analyte are separated within said cavity.
 8. A method of forminga polymer electrolyte membrane fuel cell (PEMFC) sensor comprising:providing an electrolyte, an anode, and a cathode, wherein said anodeand said cathode have a variable loading of catalyst thereon; and hotpressing said electrolyte, said anode and said cathode together to formthe PEMFC sensor.